Intrinsic defects in cubic NaNbO3

Fermi level engineering of antiferroelectrics

2022/07/28 by

Antiferroelectric NaNbO3 is a candidate material for application in high-energy density dielectric capacitors. In this context, various doping strategies have been used for installing the desired narrow double P–E loop behavior in this lead-free material. However, controlled doping requires a detailed understanding of the type and population of intrinsic defects.

Local structure around the vacancy complex in NaNbO3 after relaxation.

The formation energies, equilibrium defect transition levels, and doping behavior of intrinsic defects in cubic NaNbO3 have been determined by means of Density Functional Theory (DFT), with a hybrid exchange-correlation functional (HSE06).

Nb vacancies are found to have high formation energies, while the formation energies of Na and O vacancies are relatively small. In Na-poor/O-rich conditions, the formation energies of Na vacancies are the lowest in energy, while in all other conditions, the formation energies of O vacancies are found to be the smallest. The calculation of the equilibrium electron chemical potential shows that in Na-poor/O-rich conditions the semiconductor tends more toward being ??-type, while in all other conditions, the electron chemical potential is closer to the CBM, resulting in an ??-type semiconductor. Moreover, the study of the dependency of the defect concentrations and the electron chemical potential as a function of the oxygen partial pressure shows that the donor-like behavior of the system is attenuated when the oxygen partial pressure increases. Finally, the binding energy of the complex formed by two Na vacancies and one O vacancy is found to be negative, which indicates that defect complexes might be present in relatively large concentrations and, therefore, affect the P–E loops. We note that previously only dopant-vacancy complexes were proposed as an influencing factor. This calls for a deeper investigation on defect complexes, in particular, on charged complexes formed by intrinsic and extrinsic defects, which by introducing dipoles in the system can be decisive for the AFE properties through introducing dipoles in the system.

Journal of Applied Physics 131, 124106 (2022)